Speed control system



April 0, 1946. J. E. REILLY 2,399,387

SPEED CONTROL SYSTEM Filed April 27, 1944 I32 I34 I86 Fig.2?

l/i ?6 WITNESSES: [44 INVENTOR ATTORNEY 624 j v Ja 5.5a), ga y Patented Apr. 30, 1948 UNITED STATES PATENT OFFICE SPEED CONTROL SYSTEM Jacki Reilly, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporationof Pennsylvania Application April 27, 1944, Serial No. 533,080

4 Claims.

' wave rectifier discharge valves for controlling the direction and speed of a reversible motor.

Other objects this invention will become apparent from thei'ollowing description when taken in conjunction with the accompanying drawing, in which:

Figure 1 is a diagrammatic view of a speed control system embodying the teachings of this invention, and I Fig. 2 is a diagrammatic view of a speed control system embodying the teachings oi this invention as applied in controlling the position of an electrode of an arc furnace.

Referring to Fig. 1 of the drawing, thisinvention is illustrated by reference to a motor I ll,

through the transformer 52, the phase shifting circuit l4, and the grid transformer 58, the terminals of the secondary winding 58 of which Justable lead II, to the midor center tap of the the speed of which is to be controlled. The mo- I tor ll comprises the armature windings I2 and the field windings H which are connected through a rectifier bridge circuit it to the secondar winding 01' a transformer It, the primary windings oi which are connected across a source of supply of alternating current represented by conductors 20 and 22.

In order to control the speed of the motor it, suitable discharge devices or electric valves 24 and 28 are connected in the armature winding circuit so as to provide for complete rectification oi the alternating current from the source of supply 20 and 22. Thus the anodes 28 and III of the discharge devices 24 and 26 respectively, are connected to the terminals of the secondary winding 32 of a transformer 34, the primary winding of which is connected across supply conductors 2| and 22 while the cathodes 38 and are connected by conductors 4| and 42, respectively. to conductor 44 and one side of the armature windings l2, the. other side of the armature winding being connected by a conductor 46 to the mid or center tap of the secondary winding 32.

As illustrated, provision is madeto control the bias of grids l8 and SI of dischargedevices 24 and 28, respectively, in accordance with variations in the speed of the motor It. An alternating-current potential is normally supplied to the grids 48 and SI from the source of supply 20 and 22 are connected through resistors and 62 to grids 4i and N. respectively. The discharge devices 24 and 26 are of the well known thyratron type, the conductivity thereof being controlled by the grid-cathode potential which is made up of an alternating-current potential superimposed on a direct-current biasing potential so that the resultant grid-cathode potential becomes more a positive than the critical potential of the thyratron to render the device conductive. Thus control of the conductivity oi the discharge devices 24 and 28 is obtained by varying the direct-current biasing potential. For this purpose a resistor 84 is disposed to be connected througha full wave rectifier discharge valve 86 to the source of supply 20 and 22, a part of the resistor 64 being connected by conductor to conductors to and 42 and the cathodes I8 and 30, respectively, of

the discharge devices 24 and 28, and by the adsecondary winding ll of the grid transformer 56.

The full wave rectifier discharge device 88 comprises. two anodes l2 and 14 connected by con ductors It and II, respectively, to the terminals of the secondary winding ll of the transformer 82, the primary winding of which is connected across the supply conductors 20 and 22 and a single cathode l4 connected by conductor I8, self biasing resistor I, conductors 2, 88 and 68 to one end of the resistor 64. The other end of the resistor 64 is connected by conductor 90 to the mid or centertap of the secondary winding 80 of transformer l2. Two grids 92 and 94 are disposed within the rectifier discharge device 66 for controlling the conductivity thereof.

In order to control the bias of the grids 92 and 94 in accordance with the speed of the motor I0, an exciter generator 88 is disposed to be driven by the motor it. The generator 96 comprises the armature, windings 98 connected by conductor I, through resistor 2 and conductor 86 to the, cathode 84 of the rectifier discharge valve 86 and by conductor I00 through resistor I02 to the grids l2 and St. A shunt field winding IE4 is connected across the armature wind-" conductivity of the discharge devices 24 and 28.

If the speed of the motor I should vary from the predetermined speed as, for example, if the speed should be decreased, the output voltage of the generator 86 is decreased whereby the bias of the grids 82 and 84 of the rectifier discharge device 66 becomes more negative, the grid circuit of such rectifier device extending from the grids 92 and 94 through the resistor I02, conductor I00, armature windings 88, conductors 88 and 2, resistor I and conductor 88 to the cathode 84, As the grids 82 and 84 become more negative, the flow of current through the resistor 54 is decreased, the circuit therefor extending from the midtap of the secondary winding 80 of the transformer 82 through conductor 80, resistor 64, and conductors 68, 88 and 2, resistor I and conductor 86 to the cathode 84 and anodes 12 and 14 through conductors 15 and 18, respectively, to the terminals of the secondary winding 80.

As the current flowing through the resistor 84 is decreased, the direct-current biasing potential applied to the discharge devices 24 and 28 be comes less negative, the grid control circuits therefor extending from the grids 48 and 50 through resistors 50 and 82, respectively, to the terminals of the secondary winding 58 of transformer 58, the center tap of the secondary winding 58, flexible lead 10, part of the resistor 54,

and conductor 68 through conductors 40 and 42 to the cathodes 38 and 38, respectively. As the direct-current biasing potential becomes less negative, the resultant grid-cathode potential of the discharge devices 24 and 28 is so raised that the resultant grid-cathode potential rises above the critical potential of the devices earlier in the half-period, and the discharge devices 24 and 28 become alternately conducting sooner in the half-cycle with the result that the average current conducted becomes greater. Thus, as the biasing potential of discharge devices 24 and 28 becomes less negative, more current flows through the armature windings I2 of the motor l0 through the circuit extending from the center tap of the secondary winding 32 of transformer 34, through conductor 48, the armature windings I2, conductor 44 to and through conductors 40 and 42 to cathodes 36 and 38, respectively, anodes 28 and 30, respectively, to the terminals of the secondary winding 32. This increase in the current flowing through the armature windings I2 increases the speed of the motor I0 to the predetermined required value.

As the speed of the motor approaches the predetermined required value, it is apparent that the biasing potential of the rectifier discharge device 66 is changed in accordance with the output of the generator 88 which is driven by the motor I0. Thus, as the speed of the motor I0 is raised, the output of the generator 98 is also raised so that the biasing potential on the grids 92 and 94 become less negative to permit more current to be conducted by the rectifier discharge device 66 with the result that more current flows "through the resistor 84 to render the direct-current biasing potential in the grid-cathode circuit of the discharge devices 24 and 26 more negative with the result that less current is alternately conducted by the devices 24 and 28, thereby controlling the current flow through the armature windings I2 of the motor I0 to prevent an overrun of the predetermined required speed at which it is desired to operate the motor I0. Of course, if for any reason the speed of the motor I0 should increase above the predetermined value, the same operation of the system to decrease the current flow through the armature windings is obtained to effect a decrease in the speed of the motor I0.

Referring to Fig. 2, the system of this invention is illustrated as applied to an arc furnace I20 for controlling the position of the electrode I22 thereof with respect to a metal bath I24 in the receptacle I26. As illustrated, the arc fur nace utilizes three electrodes I22, I28, and I30 connected by conductors I32, I34 and I35, respectively, to a source of supply (not shown). Since each electrode I22, I28 and I30 is disposed to be operated in response to predetermined current flow in the electrode and the arc potential thereof, the control system for only the electrode I22 is illustrated, it being understood that similar control systems are to be provided for the other electrodes I28 and I30.

In this embodiment, a major part-of the control system comprises the same system and apparatus utilized in the system of Fig. 1, and like numerals are employed for indicating the same parts. Thus in the embodiment of Fig. 2 the motor I0 is utilized for controlling the position of the electrode I 22, the electrode I22 being connected to be raised or lowered by operation of the motor I0. For this purpose a winding drum I38 is disposed to be driven by the motor I0, a flexible cable I40 being connected to the electrode I22 and disposed over a pulley I42 to be wound upon the drum I38. Since it is desired that electrode I22 be raised or lowered, as the case may be, the motor I0 comprises a reversible motor in this embodiment.

As illustrated, two sets of discharge devices 24-45 and 25-21 are utilized for controlling the direction and speed of operation of the motor I0. The discharge devices 24, 28, 25 and 21 are provided with anodes 28, 30, 28 and 3I, respectively, connected to the terminals of the secondary windings 32 and 33, respectively, of the transformers 34 and 35, respectively, the primary windings of each of which are connected across the supply conductors 20 and 22. Each of the discharge devices 24, 25, 25, and 21 are also provided with cathodes 38, 38, 31 and 38, respectively, the cathodes 38 and 38 being connected through conductors 40 and 42, respectively, to conductor 44, the armature windings I2 of the motor I0, conductor 48, to the center tap of the secondary winding 32 of transformer 34, whereas the cathodes Hand 38 are connected by conductors H and 43, respectively, to conductor 48, conductor 45, the armature windings I2 of motor I0, conductor 44, conductor 88, and conductor 41 to the center tap of the secondary winding 33 of transformer 35. Each of the discharge devices 24, 26, 25 and 21 is provided with a grid, namely, 48, 50, 49 and 5I, respectively. An alternating current potential is supplied to the grids 43 and 50 from the supply conductors 20 and 22 through transformer 52 the phase shifting circuit 54, and the grid transformer 56, the terminals of the secondary winding 58 or which are connected through resistors and 82 to grids 48 and 58, respectively. Likewise, an alternating current potential is supplied to the grids 48 and 5t from supply conductors 20 and 22 through transformer 53, the phase shifting circuit 55 and the grid transformer 56, the terminals of the secondary winding 58 of which are connected through resistors GI and 83 to grids 48 and 5|, respectively.

In order to control the grid-cathode potential of the discharge devices 24-26, 25-21, a resistor 64 is disposed in circuit relation with the grids assess? 48 and 80 and a resistor 88 is disposed in circuit relation with the grids 48 and II, a part of the resistor 84 being connected to the conductors 40 and 42 and the cathodes 88 and 88, respectively, of discharge devices 24 and28 by the conductor 88, a flexible lead connecting some intermediate point of the resistor 84 to the center tap of the secondary winding 88 of the grid transformer 88. Likewise a part of the resistor 88 is disposed to be connected by conductor 88 to conductors 4i and 48, and cathodes 81 and 88, respectively, of the discharge devices and 21, an intermediate tap of the resistor 85 being connected by a flexible lead H to the center tap of the secondary winding 88 of grid transformer 51.

In order to control the direct-current biasing potential for the two sets of discharge devices 24-28 and 28-21, full wave rectifier discharge valves 88 and 81 are disposed to control the current fiow through the resistors 84 and 88, .respectively. The rectifier discharge devices 88 and 81 are provided with anodes 12-14 and 18-18, respectively, connected to the terminals of the secondary windings 80 and 8|, respectively, of

transformers 82 and 88, respectively, the primary windings of which are connected across the supply conductors 28 and 22.

The rectifier discharge device 88 is also provided I with a cathode is connected by conductor as,

self-biasing resistor I, conductors 88, 44 and 88 to one end of the resistor 84, the other end of the resistor 84 being connected by the conductor 80 to the center tap 'of the secondary winding 80 of transformer 82. In a similar manner, the rectifier discharge device 81 is also provided with a cathode 88 connected by conductor 81, selfbiasing resistor 8, conductors 48 and 88 to one end of the resistor 88, the other end of the resistor 88 being connected by conductor 8i to the center tap of the secondary winding 8| of the transformer 88. Likewise, each of the rectifier devices 88 and 81 is provided with two grids for controlling the conductivity thereof, the rectifier device 88 having grids 82 and 84 and the rectifier device 81 being provided with grids 88 and 88.

The grid biasing potential of the rectifier devices 88 and 81 is controlled in response to the current flow through the electrode I22 and the are potential thereof. Thus the grid-biasing circuit of the rectifying discharge device 88 extends from the grids 82 and 84 through the grid resistor 88, conductor I08, resistor I08, resistor I I0, conductor II2, self-biasing resistor I, and conductor 88 to the cathode 84. The resistors I08 and 0 are control resistors and are disposed to have a direct-current voltage thereacross proportional to the are potential and the flow of current through the electrode I22, respectively. In order to impress the direct-current voltage proportional to the are potential across resistor I08, the resistor I08 is connected across a rectifying bridge II4 which is connected through the transformer Hi, the terminals of the primary winding of which are connected by conductor I I8 to the receptacles I28 and by conductor I44 to conductor I82. The control resistor H0 is connected across a rectifying bridge circuit I48 which is connected across a transformer I48, the primary windings of which are connected to the current transformer I80 on the supply conductor I22. The transformers H8 and I48 arepreferably of a one to one ratio and are employed primarily to prevent sneak circuits or effects on 3. the grids of the rectifier discharge devices 88 and 81.

It is to be noted that the rectifying bridges I48 and II4 are connected in opposition so that the direct-current voltages across resistors I08 and I I0 are of opposite polarity, so that for any variation in the fiow of current through the electrode and in the are potential, the differential of the direct-current voltages across resistors I08 and H0 controls the direct-current biasing potential on the grids 82 and 84 of the rectifying discharge device 88.

The grids 88 and 88 of the rectifying device 81 are likewise connected through the grid resistor I8I, conductor I81, control resistor I08, control resistor III, conductor III, a self-biasin resistor 8, and conductor 81 to the cathode 85 of the rectifier discharge device 81. In order to impress a direct-current voltage across resistor I08 which is proportional to the are potential, the resistor I08 is connected across the output terminals of a rectifying circuit III, the input terminals of which are connected by conductor II8 to the receptacle I28 and conductor I44 to the conductor I82. Similarly, the control resistor III is connected across the output terminals of a rectifying circuit I41, the input terminals of which are connected. across the current transformer I80 on the conductor I 32 whereby the direct current voltage impressed across resistor II I is proportional to the current flowing through the electrode I22. As in the previous case. the rectifying circuits II! and I41 are so connected across the resistors I08 and III, respectively, that the direct-current voltages impressed thereacross are of opposite polarity for controlling the direct-current biasing potential of the grids 88 and 88 of the full wave rectifier discharge device 81. I

In constructing the system, the resistors I and 8 are selected so as to assure a negative bias on the grids of the rectifier discharge devices 86 and stantialiy zero. The resistors 84 81 when the electrode I22 is in an ideal operating position and the direct-current voltages across the control resistors are so balanced that the differential of the direct-current voltage is suband 65 in the devices 24-26 adjusted by adcircuit with the sets of discharge and 28-21, respectively, are so Justing the flexible leads 18 and H, respectively, that the drop across the portion of the resistors 84 and 88 in circuit with the grids of the sets of discharge devices 24-28 and 28-21 gives a sufficient dead zone to prevent simultaneous firing of both sets of the discharge devices.

In operation, assuming that the system is energized, the electrodes I22, I28 and I may be positioned with respect to the metal I 24 in the receptacle I28 by any suitable means such as by manual operation or automatically. Assuming that the initial positioning of the electrodes is to be obtained automatically, as soon as the system is energized and power is to be supplied to the arc furnace I20, 9. potential exists from the electrode I22 to the metal I24 -in the receptacle. This potential is at a maximum and since the electrodes I28, I80 are not as yet adjusted, there is no current fiow. The high value of the are I potential impresses a high value of direct-min rent voltage across each of control resistors I08 and I08, and since current is not flowing through the electrode I22, the direct-current voltage across resistors H0 and III proportional to the flow of current in the electrode is zero. The direct current voltage across resistor I08 places a large negative bias on the grids 03 and SI oi rectitler device 01 whereas the direct-current voltage across resistor I08 places a less negative or more positive bias on the grids 02 and 04 of the rectifier discharge device 08.

The bias thus impressed on the grids l2 and I4 is such that the rectifier discharge device 04 is rendered conductive with the result that current flows from the center tap of the secondary winding 80 through conductor 00, resistor 04, conductors 68, 44, 88, self-biasing resistor I, conductor 86, cathode 84 and anodes I2 and I4 through conductors 16 and I8, respectively, to the terminals of the secondary winding 80 of the transformer 82. Since the direct current voltage proportional to the are potential is at a maximum, the conductivity of the rectifier discharge device 60 is also a maximum and maximum current flow-s through the resistor 64. The fiow oi the current through the resistor 64 renders the direct-current biasing potential on the grids 48 and 50 of the discharge devices 24 and 20 more negative so that the grid-cathode potential of those devices is so lowered below the critical potential 01' the devices and the devices are blocked with the result that no current fiows therethrough to the motor I0.

At the same time, the large direct current voltage proportional to the are potential impressed across resistor I09 cooperates with the self-biasing resistor '3 to impress a more negative biasing potential on the grids and prevent the rectifier 01 from conducting. By preventing the firing of the rectifier 01 there is no current fiow through the resistor 85 in the anode-cathode circuit of the rectifier i1 and with the direct-current biasing potential thus reduced the resultant grid-cathode potential of the devices 25 and 21 rises above the critical potential or the devices and the devices 25 and 21 become highly conductive to pass current to the motor III. This conducting circuit may be traced from the center tap of the secondary winding 33 of transformer 25, through conductors 41, I8, and 44, armature windings I2 of the motor I0, conductor 40, conductor 40, to and through the parallel conductors H and 42 to the cathodes I1 and 39, respectively, of the devices 24 and 21' and the anodes 20 and SI, respectively, to the terminals of the secondary winding 22, to so energize the motor I as to effect the operation thereof to lower the electrode I22. Since the other electrodes I28 and I30 are as yet not in contact with the metal bath I24, the electrode I22 is lowered to a position where it engages the metal in the receptacle I26.

In lowering the electrode I22, it is apparent that the potential between the electrode I22 and the metal I24 decreases with the result that the direct-current voltage across resistors I00 and I09 is so decreased that the bias on the grids of the rectifier devices lit; and G1 is respectively rendered more negative and less negative. By rendering the biasing potential of the grids of the rectifier device 68 more negative, the current flow through the resistor 54 for controlling the directcurrent biasing potential of the devices 24 and 26 more nearly approaches the value where the discharge devices 24 and 28 can be rendered conductive.

As soon as the other electrodes I28 and I30 are actuated in a similar manner to a point where a circuit is completed through the electrodes I22, I28, and I30 in conjunction with the metal bath I24, maximum current flows through the electrade with the result that a direct-current voltage proportional to the current is imposed across control resistors H0 and III with the result that the diflerential or the direct-current voltages across control resistors III-I04 and III-I00 is such as to place a more negative bias on the grids of the rectifier discharge device 00 and a less negative bias on the grids of the rectifier device \l.

The less negative or more positive bias on the grids 01 and 05 of rectifier device 01 renders the rectifier conducting so that current flows through the circuit which may be traced from the center tap of the secondary winding 8: of transformer 02, through conductor II,- resistor 65, conductor a, conductor 48, self biasing resistor 3, conductor 01, cathode l0, and the anodes 12 and 15 of rectifier discharge device 01, through conductors 11 and 10, respectively, to the terminals of the secondary winding 8|. The current flowing through that part of the resistor 65 in the grid cathode circuit or the discharge devices 25 and 21 is such as to render the direct-current biasing potential on the grids 40 and I or the devices 25 and 21, respectively, more negative to effectively block the firing of said devices and prevent the fiow of current therethrough to the motor I0.

On the other hand, the change in the biasing potential of the grids 02 and 84 of the rectifier discharge device It is such as to eiiectively block the passing of current by the rectifier device it with the result that the current flow through the resistor 64 is so decreased or becomes zero so that the direct-current biasing potential on the grids 44 and 50 of the discharge devices 24 and 20, respectively, becomes less negative or more positive and the devices 24 and 2B become conducting to pass current to the motor I0. This conducting circuit may be traced from the center tap of the secondary winding 22 of transformer 34 through conductor 48, armature winding I2 of the motor I0 and conductor 44 to the parallel conductors 40 and 42 and the cathodes 28 and 20, respectively, anodes 24 and 20, respectively, of the devices 24 and 28, respectively, to the terminals of the secondary winding 22 to so energize the motor I0 as to eflect an operation thereoi in a direction to raise the electrode I22.

The electrode I22 is thus raised to a position for ideal operation of the arc Iurnace in which position the direct current voltage across resistors H0 and III which is proportional to the current fiow through the electrode I22 and the direct current voltage across resistors I00 and III! which is proportional to the are potential are balanced so there is a zero differential of direct current voltage acros the series-connected control resistors I08 and III) arid the series-connected control resistors I09 and III. With the differential voltages zero as described, the selfbiasing resistors I and 3 function to maintain suflicient bias on the grids of the rectifier devices 06 and 61 whereby they are sufllciently conducting to pass sufficient current through resistors G4 and 85 that the direct current biasing potential on the grids of discharge devices 24, 2B, 25 and 21 is sufiicient to block the devices from passing current to the motor I0. It i also noted that by correctly setting the position or the variable leads I0 and II with regard to the resistors 64 and 05, respectively, a dead zone is provided for preventing the simultaneous operation of the two sets of discharge devices 24-25 and 25-41.

It the ideal operating condition for the arc furnace i changed in any manner whatsoever,

aseassv the balance of the direct-current voltage across the control resistors is upset with the result that the bias oh the grids of the rectifier discharge devices 68 and l is changed to effect the operation of the motor II in a predetermined direction depending upon the change in the ideal operating conditions. If, for example, the electrode I22 should come in contact with the metal I24, as by reason of a cave-in of the metal, the arc potential i reduced to a minimum and the current flowing through the electrode is increased to a maximum, with the result that the difierential or the direct-current voltages across the resistors I08 and III is such as to place a more negative bias on the rectifier discharge device 66 and the difierential of the direct current voltages across resistors Ill and III is of such polarity as to render the grids of the rectifier device 61 more positive and thereby render the discharge device 61 conductive. Again, as in the earlier illustration, when the discharge device I! is rendered conductive, the flow oi current through the resistor so changes the gridcathode potential of the devices 2! and 21 as to effectively block them, whereas the decrease in the direct current flowing through the resistor 64 so changes the direct-current biasing poten- 'tial as to render the grids of the devices 24 and 20 less negative whereby they are rendered conducting to operate the motor ll to raise the electrode I22.

A will be appreciated, any unbalance in the direct-current voltages across the control resistors in response to a change in the ideal operating conditions for the electrode 22 efiects an immediate change in the current flowing through the armature I2 of the motor I0, so that in the case or the system illustrated in Fig. 2 direct control oi the direction of operation of the motor II is obtained, and, as the motor is operated to move the electrode I22 to its ideal operating position, the speed of the motor is so changed as to decrease the speed or the movement of the electrode and prevent an overrun in the positioning or the electrode.

The system of this invention is very sensitive, giving quick response in the control oi the motor and, by utilizing the full wave rectifier discharge device in the manner described, a sim-' plification of the circuits is obtained. The full wave rectifier device utilized performs the functions 01' rectifying, amplifying, and controlling the system. As illustrated, the system is adapted to wide use, since it is capable of maintaining a constant speed as in the embodiment of Fig. 1, or of controlling the direction of rotation and the speed of a reversible motor as in the embodiment of Fig. 2.

Although this invention has been described with reference to particular embodiments thereof. it is, of course, not to be limited thereto except insofar a i necessitated by the scope ofthe app nded claims.

I claim as my invention:

1. In a speed control system, in combination, a motor the speed of which is to be controlled. a pair of electric valves disposed to connect the motor to a source of alternating current and to control the speed of the motor, the electric valves having grids to be utilized for controlling the conductivity thereof, a control circuit disposed for controlling the bias imposed on the grids, the control circuit including a resistor connected in circuit with the grids for controlling the bias thereof, means including a full-wave rectifier dis- 7' charge valve having control grids therein for supplying a variable direct current to the resistor, and means comprising a source of control voltage variable in response to changes in the speed of the motor disposed to control the bias On the control grids of the rectifier discharge valve and thereby control the conductivity of the pair of electric valves.

2. In a control system for regulating the feeding of an electrode for producing and maintaining an electric arc, the combination comprising, a motor the speed oi which is to be controlled, a pair of electric valves disposed to connect the motor to a source of alternating current and to control the speed oi the motor, the electric valves having grids to be utilized for controlling the-conductivity thereof, a grid control circuit including a resistor for the electric valves, a full-wave rectifier discharge valve having control grids therein to' be utilized for controlling the flow 01 direct current through the resistor to control the bias of the grid of the electric valves, and a pair of sources of control voltages variable in opposite senses or controlling the bias imposed on the control grids of the rectiiier discharge valve and thereby control the conductivity of the electric valves and the speed of the motor.

3. In a control system for regulating the feeding 0! an electrode for producing and maintaining an electric arc, the combination comprising, a motor the speed of which is to be controlled, a pair 01 electric valves disposed to connect the motor to a source oialternating current and to control the speed of the motor in the one direction, the electric valves having grids to be utilized for controlling the conductivity thereof, a control circuit disposed i'or controlling the bias imposed on the grids, the control circuit including a resistor connected in circuit with the grids, a-lullwave rectiiier discharge valve having control grids therein to be utilized for controlling the now of direct current through the resistor, and means including a pair of sources of control voltages variable in opposite senses ior controlling the bias 0! the control grids and thereby control the speed of the motor.

4. In a control system for regulating the feeding of an electrode for producing and maintaining an electric arc, the combination comprising. a reversibie motor the direction and speed of which is to be controlled, a plurality 0! pairs of electric valves disposed to selectively connect the motor to a source 0! alternatin current and control the direction and speed or the motor, each of the electric valves having a grid to be utilized for controlling the conductivity thereof, a grid control circuit including a resistor for each pair of the electric valves, each of the resistors having a lullwave rectifier discharge valve connected in circuit relation therewith for controlling the now of direct current therethrough to control-the bias of the grids of the pair of electric valves, each of the rectifier discharge valves having a pair of control grids, and a plurality of pairs of sources of control voltages, each pair oi sources being connected in circuit relation with the control grids of an associated rectifier discharge valve, the pairs of sources being of opposite polarity when the pairs are unbalanced to render only one oi the I rectifier discharge valves conducting at a time to thereby selectively control the conductivity of the pairs of electric valves and the operation oi the motor.

JACK E. REULY. 

